Genie Control of Development 307 



naturate RNA. Such a treated piece transplanted into a normal host 

 becomes revitalized. It can be observed that the new synthesis of 

 RNA in the fragment attached to a normal one begins next to the 

 normal cells and proceeds from there on. But this might also be due 

 to a difiFusible substance needed, and does not necessarily prove an 

 actual intercellular migration of microsomes. The main function of 

 RNA is supposed to be protein synthesis (refer to Caspersson) and, 

 therefore, Brachet and followers assume that the microsomes are the 

 seat of protein synthesis. Experiments with tagged amino acids are in 

 favor of this assumption. 



In the description of the RNA behavior as derived from the work 

 of Brachet, Dalq, and many others, we omitted reference to nuclear 

 activity, which of course is the most interesting point for the 

 geneticist. Brachet considered quantitative changes vdthin the nucleus 

 by measuring the presence of suLfhydril groups. He found their amount 

 variable in different tissues in course of development. Krugelis (1946) 

 found a striking increase in the amount of alkaline phosphatase in 

 the nuclei (and also the cytoplasm) after gastrulation. Very con- 

 spicuous intranuclear changes during development were claimed by 

 Lison and Pasteels (1951). We studied before the facts concerning 

 the constancy of DNA in all nuclei of a species when measured by 

 optical methods. The Belgian authors found that this is no longer true 

 after the 4-blastomere stage in the sea urchin egg, the mesomeric 

 quartette having more than the other cells. Later, other and more 

 remarkable differences appear, not following this increase rule; an 

 increase in DNA was especially noticeable in regions of quick cell 

 division. The students of the endomitotic DNA quantities maintain 

 that this result is due to a faulty technique. Whether this is true or 

 not, we may say that thus far a definite relation between intranuclear 

 biochemistry and morphogenesis has not been established. 



From another point of view, Brachet attacked the problem of the 

 relations between cytoplasmic RNA and the nucleus during morpho- 

 genesis. He starts with the fact that development in most anuran 

 species hybrids stops at the beginning of gastrulation. He concludes 

 that this is due to an inhibition of RNA and protein synthesis by the 

 presence of a foreign nucleus. If this were so, the dependence of the 

 RNA activity upon the nucleus would be proved. When the so-called 

 organizer of such a hybrid is transplanted to a normal embryo, it starts 

 proper differentiation, is revitalized. It seems, further, that at the time 

 of the block, RNA-rich nucleoli are produced and that, simultaneously, 

 the "chromatin" is unusually rich in RNA. Thus the anomaly seems to 



